Perfume can seem strong when first applied and almost disappear from awareness a few minutes later. It is easy to assume that the fragrance has already dispersed. More often, the olfactory system has reduced its response to a steady stimulus: the odour remains in the air while its perceived intensity falls.
Research commonly discusses two related processes, olfactory adaptation and habituation. Adaptation refers to neural changes caused by sustained or repeated stimulation and may occur at the olfactory epithelium or at higher levels. Habituation places more emphasis on the brain and behaviour giving less attention to a familiar odour that no longer demands a response. In ordinary experience, the two are difficult to separate completely. Reviews show that concentration, exposure duration and recovery time all affect the decline, while adaptation to one odour does not always generalise equally to another.
Consider applying perfume in a bedroom and then remaining in the same scented environment. Your own experience fades, but someone entering the room has not had the same continuous exposure and may still notice it clearly. The smell may also become prominent again after you step outside briefly and return. Olfaction is especially responsive to change rather than maintaining an equally strong response to a constant background.
“I can’t smell it” therefore does not directly imply that the fragrance is gone, and repeated reapplication may become excessive for other people. This rapid, odour-specific decline is a common adaptive effect, but it does not explain every persistent or sudden change in smell. Controlled studies use particular odours, concentrations and exposure periods; mixtures, ventilation and individual differences make real settings more variable.
Sources: https://pubmed.ncbi.nlm.nih.gov/10944515/
https://pubmed.ncbi.nlm.nih.gov/28408237/